This invention relates to an improved process for the production of urethanes (carbamic acid esters). More specifically, organic nitro compounds and organic compounds containing at least one hydroxy group are reacted in a hydrocarbonylation reaction with a reaction gas containing both carbon monoxide and also hydrogen in a high concentration in the presence of noble metals of the Eighth Secondary Group of the Periodic System and certain co-catalysts to form urethanes.
The production of urethanes from organic nitro compounds, organic hydroxyl compounds and carbon monoxide in the presence of noble metal catalysts is already known. German Offenlegungsschriften Nos. 28 19 826 and 29 03 950, for example, describe processes by which urethanes can be produced in very good volume/time yields through the use of special co-catalysts, such as iron oxychloride or combinations of iron oxides or iron oxide hydrates with activating chlorides, in addition to tertiary amines. Although the urethane synthesis described in these publications affords the possibility of phosgene-free isocyanate production by thermal splitting of the urethanes, not one of those processes has been adopted for industrial scale production. One reason for this non-use on a commercial scale could be that the cost of carrying out these processes is too high to be commercially practical.
Japanese Patent Application No. 53/79076 filed June 29, 1978 and published under the number 55-7227 (1980) discloses that the formation of urethanes from the above-mentioned starting materials in the presence of palladium catalysts is accelerated by the presence of small quantities of hydrogen. This disclosure also emphasizes that the yield of urethane decreases if there is more than 6% volume of hydrogen in the reaction gas. Further, from the Examples of this Japanese publication it appears that the process disclosed therein is based on the use of selenium as the main catalyst component. In the process of the present invention described in more detail hereinafter however, the reaction is carried out in the absence of selenium and hydrogen is present in considerable quantities and enters stoichiometrically into the urethane-forming reaction. Japanese No. 55-7227 also does not use the co-catalysts required in the process of the present invention.
Japanese Patent Application No. 56-68249 filed May 8, 1981 and published under the number 57-185253 in 1982 describes the use of mixtures of carbon monoxide and hydrogen in the synthesis of urethanes from aromatic nitro compounds in the presence of amino compounds which correspond to the nitro compounds and which are simultaneously reacted to form urethanes. In this disclosed process, (which does not employ the co-catalysts required in the present invention), uneconomically large quantities of noble metal catalysts must be used, as is evident from the general description and the Examples given therein.